1. Field of the Invention
This invention relates to a toner density control apparatus for a reproducing apparatus.
2. Description of the Prior Art
An electrophotographic reproducing apparatus is operated as shown in FIG. 3. Namely, an original (not shown) placed on a platen glass 1 is scanned by a light source 2 in the direction of an arrow a, and the light reflected on the original is guided to a photosensitive drum 9, which is charged with a high voltage in advance by a charging electrode 8 and rotated in the direction of an arrow b synchronously with the scanning of the original, by an optical system consisting of mirrors 3-6 and a lens 7, to form a latent image on the drum 9. This latent image is developed with a toner by a developing unit 10 in a subsequent process, and the resultant toner image is transferred by a transfer electrode 11 to transfer paper fed in the direction of an arrow c. The resultant transfer paper is separated from the photosensitive drum 9 by a separating electrode 12 and carried to a thermal fixing unit 14 by a transfer belt 13. The image on the transfer paper is fixed in the thermal fixing unit 14, and the resultant transfer paper is discharged therefrom. Reference numeral 15 denotes a cleaning unit for removing the residual toner from the photosensitive drum 9, 16 a lamp for eliminating the unnecessary charge from the surface of the photosensitive drum 9 electrically charged by the charging electrode 8, so as to prevent the wasteful consumption of the toner, and 17 a toner density detecting sensor.
As the toner in the developing unit 10 is fed to the photosensitive drum 9, the amount of toner in the unit 10 decreases, and, when the decrease in amount of the toner has continued, the image becomes thin. To prevent this inconvenience, the toner density is detected by the sensor 17 to control the quantity of the toner in the developing unit 10.
This toner density control operation is carried out by scanning an image (in reference black color) on a reference density plate 18, which is provided in advance on the side of the starting end of the platen glass 1, prior to the scanning of the original, forming a latent image and developing the same, detecting the density of the developed image by the sensor 17, and opening a valve 10b in a toner hopper 10a in the developing unit 10 predetermined times or for a predetermined period of time when the density of the image is not higher than a reference level, to supply the toner to a sleeve 10c.
FIG. 4 shows a toner density control circuit. The sensor 17 consists of a reflecting photocoupler composed of an LED 17a as a light-emitting element, and a phototransistor 17b as a light-receiving element. Reference letters VR1, VR2 denote variable resistors for regulating the electric current flowing through the LED 17a, Q1, Q2 transistors for regulating the voltages at both ends of one variable resistor VR2, Q3 a transistor for amplifying an output from the phototransistor 17b, R1 a bias resistor for the base of the transistor Q2, R2, R3 resistors for converting an output current from the transistor Q3 into a voltage, and TH a temperature compensating element.
If the electric current flowing through the LED 17a in this circuit is constant (the light-emitting rate is constant), the rate of reflection of the light on the toner is low when the toner density to be detected is high. Accordingly, an output current from the phototransistor 17b becomes low, so that the voltage at an output terminal 21 also becomes low. When the toner density is low, the voltage at the output terminal becomes high in contrast to the above-mentioned case.
A comparator (not shown), to which a voltage corresponding to a reference level of the toner density is applied as a comparative reference level, is connected to this output terminal 21. Owing to this arrangement, when a voltage higher than this comparative reference level is outputted, a signal is outputted from the comparator to cause the valve 10b in the toner hopper 10a to be driven, so that the toner is supplied to the sleeve 10c. This control operation is carried out for each sheet-copying action so as to constantly maintain the toner density in a proper level.
In a conventional device, the toner density is controlled to only one certain level. However, different users prefer different image densities. In order to meet these requirements, image density selecting buttons are provided. However, these selector buttons are adapted to be pressed selectively with respect to the density of the original.
The sensor 17 referred to above is set on the upstream side of the cleaner unit 15, and it is therefore contaminated considerably with the toner. This would cause the effective rate of emission of the light from the LED 17a to decrease, or prevent a part of the light reflected on the patch image from reaching the phototransistor 17b.
If such troubles occur, it is already impossible that an accurate density of the patch image be detected. In such a case, a high toner density is detected, so that the toner is not supplementarily supplied. This causes the image density to lower.
In order to prevent such inconveniences in a conventional device, a serviceman manually washes the toner density sensor while the rate of emission of the light from the light-emitting element is regulated manually by the variable resistor VR1 during the replacement of the photosensitive drum so as to eliminate the scatter, which occurs due to the changed reflectance of the photosensitive drum, of the rate of reflection of the light thereon. Accordingly, the controlling of the toner density is not sufficiently done until the completion of the replacement of the photosensitive drum.